1. Field of the Invention
The present invention relates to a liquid crystal (LC) display device for use in audio-visual (AV) devices, office automation (OA) devices, etc., and a method for producing the same.
2. Description of the Related Art
A color LC display device for use in the display of a LC television apparatus, a lap-top type personal computer, a notebook type personal computer, or the like, typically incorporates a color filter such that the filter is formed of discrete colored layers disposed in a plane on an insulative substrate.
As a conventional color LC display device of the above-mentioned type, a device having the configuration shown in FIG. 5 is known. The color LC display device in FIG. 5 can be produced by sealing liquid crystal 24 between a color filter substrate 21 and a driving substrate 23 with a sealant 25. The color filter substrate 21 is produced by: forming respective colored layers 26 of the three primary colors, i.e., red (R), green (G), and blue (B); providing a black matrix 27 for preventing decrease in the contrast or color purity due to light leaking between the colored layers 26; smoothing the surface of the colored layers 26 and the black matrix 27 with a top coating 28; and providing a transparent electrode 29. The driving substrate 23 is produced by forming transparent driving electrodes 22 on a glass substrate 20.
The colored layers 26 of the color filter 21 may be produced by the following known methods, for example: a printing method, by which inks of RGB colors are printed on a glass substrate by means of a printing apparatus; a dispersion method, by which a ultraviolet-ray (UV ray) curable resist that contains pigments dispersed therein is applied on a glass substrate, followed by a repetition of a photolithography process for exposure through a mask and a heat curing process for each color of RGB, whereby pigment layers are formed; a dye method, by which a resist is formed through photolithography techniques for providing selective masking on gelatine so as to allow each color of RGB to be dyed with a dye; an electrodeposition method, by which an electrodeposition polymer material containing a pigment dispersed therein is applied on a substrate through electrodeposition by utilizing electrodes formed on the substrate; and a micell electrolysis method, by which a surfactant containing a pigment dispersed therein is electrolyzed by utilizing electrodes formed on a substrate.
A color LC display device is produced by attaching an active matrix substrate onto a color filter substrate, which requires positioning with high accuracy, thereby resulting in a decrease in the production yield. Furthermore, where the black matrix is designed with some margin, a decrease in the aperture ratio results.
In an attempt to solve the above-mentioned problems, Japanese Laid-Open Publication No. 64-21481 discloses a method for producing a color filter on an active matrix substrate by an electrodeposition method, which is illustrated with reference to FIGS. 6 to 8. FIG. 6 is a plan view illustrating a conventional LC display device. FIG. 7 is a cross-sectional view of the display device of FIG. 6 taken at line C--C. FIG. 8 is a cross-sectional view of the display device of FIG. 6 taken at line D--D. With reference to FIG. 7, a gate electrode 5 (of Cr, Ta, or the like) is formed on an insulative substrate 1 (e.g., glass). A gate insulation film 6 (e.g., SiN.sub.x) is formed over the gate electrode 5. A semiconductor layer 7 of an undoped amorphous silicon, or the like, is formed on the gate insulation film 6 above the gate electrode 5, consecutively after which follows the formation of an insulation layer 10 on the semiconductor layer 7. Furthermore, over the respective ends of the semiconductor layer 7, amorphous silicon (n.sup.+ type a-Si) layers 8a and 8b doped with phosphorous (P), for example, are formed. On the n.sup.30 a-Si layers 8a and 8b, a source electrode 9a and a drain electrode 9b (of Ti or the like) are respectively formed. Thus, a thin film transistor (TFT) 3 functioning as a switching element is formed. Turning to FIG. 6, the gate electrode 5 is electrically coupled to a gate bus line 15 (of Ta or the like); the source electrode 9a is electrically coupled to a source bus line 16 (of Ta or the like); and the drain electrode 9b is electrically coupled to an electrode 12b for facilitating the formation of a color filter. The electrode 12b functions as a pixel electrode. The gate bus lines 15 and the source bus lines 16, which are formed in a matrix shape, are insulated from each other by the gate insulation film 6. Turning to FIG. 7, an organic resin protection film (insulation layer) 11 is further formed on the TFT 3, the gate bus lines 15, and the source bus lines 16 (not shown). A dye resist for forming a light-shielding layer 19 is formed on the insulation layer 11 by photolithography, and dyed black to form the light-shielding layer 19. Next, a voltage is applied to the gate bus lines 15 and the source bus lines 16 so as to activate the TFT 3, thereby allowing a color filter 13 to be formed in a desired color and in desired dot positions by electrodeposition. Furthermore, an alignment film 14 (of polyimide or the like) is formed on the color filter 13 and the light-shielding layer 19. The alignment film 14 is subjected to an alignment process through rubbing or other techniques.
On the other hand, a counter substrate may be prepared by forming a counter electrode 17 on an insulative substrate 2 (e.g., glass) with an alignment film 14 (of polyimide or the like) further formed there-upon. The alignment film 14 is subjected to an alignment process through rubbing or other techniques.
After the above-described processes, the substrate 1 having the color filter layers 13 and the TFTs 3 (i.e., switching elements) formed thereupon is attached onto the counter substrate having the counter electrode 17 thereon, with spacers (e.g., plastic beads) being dispersed in the interspace for retaining a constant distance between the substrates. The liquid crystal 4 is injected between the substrates, which are sealed with a sealant. Thus, a color LC display device has been produced.
According to this method, a color filter is formed on the active matrix substrate. Therefore, the effects of the color filter are well conserved regardless of possible positioning errors during the attachment of the active matrix substrate to the counter substrate. Since there is no need to allow any extra margin for the size of the color filter as such, the pixel aperture ratio can be improved.
However, according to the method described in Japanese Laid-Open Publication No. 64-21481, a dye resist is applied on the entire surface of the substrate so as to leave portions other than the pixel electrodes open to exposure, which is followed by dying the light-shielding layer 19 black. Since this exposure process is susceptible to some positioning error of the resist pattern, a light-shielding layer tends to also form on the pixel electrodes, thereby decreasing the effective pixel area (note that the pixel electrodes are utilized to define dots of the desired color filter during the electrodeposition process).
As a method for solving the above-mentioned problem, Japanese Laid-open Publication No. 7-72473 discloses an alternative method for forming a color filter on an active matrix substrate by electrodeposition. According to this method, active elements are activated so as to form a color filter including R, G, and B dots by electrodeposition; thereafter, a light-shielding layer is formed by a rear exposure method; and a light-shielding layer is further formed by electrodeposition on any electrode on which the rear exposure method cannot form a light-shielding layer. This method provides further improvement in the pixel aperture ratio.
However, the above method, which relies on a rear exposure method to form a light-shielding layer in an attempt to achieve a 100% effective pixel area, thus requires a further step of forming a light-shielding layer by electrodeposition on the light-shielding electrodes because the rear exposure method could not form a light-shielding layer thereon. This lengthens the production process and results in an increase in cost.
Moreover, in the case where there is a disruption in the wiring coupled to a signal electrode of R, G, or B is, the above method requires a correction process involving first fixing the disruption and then forming a color filter at least in the portions which were not appropriately colored due to the disruption, this process being repeated three times. As a result, the production process of the device may be considerably prolonged.